In process automation technology, field devices are often applied, which serve for registering and/or influencing process variables. Examples of such field devices include fill level measuring devices, mass flow measuring devices, pressure, and temperature, measuring devices, etc., which, as sensors, register the corresponding process variables, fill level, flow, pressure, and temperature.
Serving for influencing process variables are actuators, e.g. valves or pumps, via which the flow of a liquid in a pipeline section, or the fill level in a container, can be changed.
Referred to as field devices are, in principle, all devices, which are applied near to the process and deliver, or work with, process relevant information.
A large number of such field devices are available from the firm, Endress+Hauser.
As a rule, field devices in modern industrial plants are connected via bus systems (Profibus, Foundation Fieldbus, etc.) with superordinated units (control systems or control units). These superordinated units serve for, among other things, process control, process visualizing, process monitoring, as well as for configuring the field devices.
In endangered areas, especially two-conductor fieldbusses, such as, for example, Profibus PA or Fieldbus Foundation busses, are applied, in the case of which both the electrical current supply of the field device as well as also data transmission from the field device (slave) to the superordinated unit (master) occur over two conductors. When the particular field device desires to transmit measurement data to the master, a corresponding electrical current modulation is impressed on the electrical current flow through the field device. Through analysis of this electrical current modulation, the master can obtain data from the field device.
In the solutions of the state of the art, the electrical current modulation occurs by means of an electrical current control transistor. For producing the electrical current modulation, the base current of the electrical current control transistor is varied corresponding to an electrical current control signal. However, the fieldbus units of the state of the art have the disadvantage that, during start-up, there is yet no base current available for the electrical current control transistor, since the fieldbus unit is not yet supplied with electrical current. Therefore, the electrical current control transistor is initially blocked. In order, nevertheless, to be able to start the field device, some solutions of the state of the art provide bypass circuits, which shunt the electrical current control transistor initially, during start-up, and supply the circuit components of the fieldbus unit, during start-up, with electrical current.
From EP 1 158 274 A1, controlled electrical current sources of two-conductor measuring devices are known, which provide the higher energy requirement required during the turning-on of a direct voltage source.
Such bypass circuits are complex and expensive. For example, appropriate circuitry must be provided, in order to remove the bypass circuit after the start-up, or at least to avoid, that the electrical current taken by the bypass circuit disturbs the electrical current modulation on the fieldbus. A further disadvantage is that the bypass circuit must be able to process the relatively high bus voltage, e.g. up to 35V according to the IEC61158-2 standard, section 12.7.2, which likewise means additional circuit complexity.